Effect of the external metal on the electrochemical behavior of M3[Co(CN)6]2 (M: Co, Ni, Cu, Zn), towards their use as anodes in potassium ion batteries

A series of crystalline-cubic Prussian blue analogues (PBA) M₃[Co(CN)₆]₂ (M=Co, Ni, Cu, Zn) were straightforwardly synthesized via the coprecipitation method as confirmed by X-ray diffraction and FTIR spectroscopy. These transition metal hexacyanocobaltates show electrochemical activity related to reversible K-ion storage between 0.05-2.6 V vs K^o/K⁺, that makes them suitable as anode-materials in K-ion batteries. The non-steady open circuit potential (OCP) observed in cells containing Cu₃[Co(CN)₆]₂ and Zn₃[Co(CN)₆]₂ electrodes suggests the presence of spontaneous redox reactions at the electrolyte/PBA interface, which leads to the loss of electrochemical activity with the time. On the other hand, the Co₃[Co(CN)₆]₂ and Ni₃[Co(CN)₆] show extended -stability periods and multiple reduction/oxidation reactions which lead to the reversible charge storage mechanism. From an in-depth electrochemical study on the Co₃[Co(CN)₆]₂-PBA, used as reference, an improvement on capacity retention is observed when the electrode is discharged at high negative potentials, 0.2 V vs K^o/K⁺, possibly due to the beneficial effect of a different electrochemical reaction on the PBA surface. The Ni₃[Co(CN)₆]₂-PBA does not follow this trend, suggesting a different kinetics for this reaction on the particle surface, avoiding to observe its favorable effect as in the case of the Co₃[Co(CN)₆]₂. This is the first time that transition metal-modified hexacyanocobaltates are reported as anode materials for K-ion batteries; particularly the Ni₃[Co(CN)₆]₂-PBA exhibits specific capacities as high as ~ 310 mA h g⁻¹ in the first discharge making it a promising material for applications in novel energy-storage technologies as K-ion batteries.